Patellar Dislocation

Overview

Patella dislocation occurs most often in young, active individuals, with the patella almost always dislocating laterally. When compared with non-surgical management, re-dislocation rates are lower following surgery (24% versus 35%) but there is no difference in recurrent instability or functional outcomes. First time patellar dislocations are usually managed conservatively, while surgery may be required for recurrent dislocations. Medial patellofemoral ligament (MPFL) reconstruction can be performed in isolation, or in combination with bony procedures (trochleoplasty and/or tibial tuberosity osteotomy) depending on the presence or absence of patella alta, trochlear dysplasia and an increased tibial tuberosity-trochlear groove (TT-TG) distance.

Anatomy & Function

The knee cap (patella) sits within the quadriceps tendon at the front of the knee and is firmly attached to the shin bone (tibia) by the patellar tendon (images 1-3). The patella protects the knee joint from direct trauma (e.g. landing on the knees) and acts as a biomechanical pulley to improve the efficiency of the quadriceps muscle as the main extensor and decelerator of the knee. The cartilage on the under surface of the patella is the thickest in the body, which reflects the large loads that are placed on the patella during activities of daily living (e.g. stairs, squatting, running).

Images 1-2: front and side views of the patellofemoral joint.

The patellofemoral compartment is part of the knee joint and is formed by the patella and the groove (femoral trochlea) at the lower (distal) end of the thigh bone. As the knee bends (flexion) from a fully straightened position, the patella descends into the trochlea, but does not fully enter this groove until approximately 20-30° of knee flexion. Within the first 20-30° of knee flexion, a significant amount of patellar stability is provided by the surrounding soft tissues (quadriceps muscle and tendon, retinaculum and surrounding ligaments) but once the patella engages with the trochlea the stability is increased by the bony ‘walls’ of the groove.

Causes

Patellar dislocation occurs when the bone is forced out of its normal position, so that the patella and trochlear surfaces are no longer in contact with one another (image 4); the patella almost always dislocates outwards (laterally). Patellar dislocations are usually caused by a non-contact twisting injury to the knee, or direct contact to the inner (medial) aspect of the patella. In some individuals, the injury can be trivial and in recurrent episodes, less force may be required to re-dislocate the bone.

Risk Factors

Numerous studies have been conducted to determine specific risk factors for patellar dislocation. Individuals with naturally lax joints (e.g. hypermobility syndrome, Ehlers-Danlos syndrome) are more susceptible to patellar instability due to the reduced support provided by surrounding soft tissues. Individuals that are under the age of 18 at the time of first dislocation are also more likely to experience further dislocations.

Specific anatomical risk factors have been associated with patellar dislocation:

Patella alta (alta = high): a naturally ‘high-riding’ patella means the patella has further to descend before engaging with the trochlea and less bony support is provided whilst the knee is relatively straight.

Trochlear dysplasia (dysplasia = abnormal development): a shallow or misshapen trochlea decreases the bony support for the patella, or a bony bump at the top of the trochlea (supratrochlear spur or boss) can prevent the patella from tracking correctly as it enters the groove.

A recent study found that 87% of individuals that suffered a first-time patellar dislocation had at least one of these anatomical risk factors: Research is currently being conducted to determine whether individual anatomy can ‘predict’ the risk of re-dislocation, but no clinically useful prediction tool is available at this present time.

Presentation

It is important to differentiate patellar and knee (tibiofemoral) dislocations; 18% of knee dislocations involve vascular injury, which can become limb or life threatening.

Patellar dislocation occurs most often in young, active individuals, especially those in the second decade of life, with the incidence decreasing with increasing age. Approximately one-third of conservatively managed patients will re-dislocate after a first patellar dislocation; after a second dislocation, more than 50% will have further episodes of instability.

The individual typically recalls a memorable incident that caused the patella to ‘pop out’ laterally. This usually occurs with the knee relatively straight, where the patellar has not yet engaged the trochlea and there is less traction provided by the quadriceps. The patella may have ‘popped back in’ by itself (images 5) or been relocated by someone else, but whilst the patellar is out of position the knee will appear deformed.

The individual is usually unable to continue the activity, or even weight bear, and may notice immediate swelling within the joint (i.e. within 2 hours of injury). Swelling that develops within this time period indicates bleeding within the knee joint (haemarthrosis); patellar dislocation is one of the most common causes of haemarthrosis. In contrast, those with naturally unstable joints (e.g. Ehlers-Danlos syndrome) may not recall a specific, traumatic event and may not experience knee joint swelling.

Pain, and occasionally bruising, is typically present on the medial aspect of the knee. Pain may also be experienced on the outside of the knee, due to an impaction of the bones as the patella relocates (images 6-7). Patellar instability may affect the alignment/tracking of the kneecap within the trochlea (images 8-9), increasing the stress on the patellofemoral joint and contributing to patellofemoral pain.

If there is a significant injury to the bone or cartilage (osteochondral or chondral defect), the individual may experience mechanical symptoms (jamming/locking), pain at a specific knee angle, or persistent swelling within the knee (effusion). Patients often complain of knee instability, which may be due to an unstable patella, quadriceps muscle inhibition secondary to pain and/or effusion, or a lack of trust in the knee.

In early presentations, individuals may find it difficult to straight leg raise and are usually reluctant to bend their knee, but this resolves in time. It is important to rule out extensor mechanism rupture and patellar fracture (see knee fracture) as these separate conditions may present with similar signs and symptoms.

Diagnosis

Various tests are used clinically in an attempt to diagnose patellar instability, including the moving patellar apprehension test (video 1), quadrant test (video 2), tenderness of the medial patellofemoral ligament (MPFL) insertions (video 2), and inverted J-sign but the diagnostic accuracy of these tests is poor, or have not been adequately investigated. Therefore, it is generally recommended that several different positive tests, in combination with the history and exclusion of other conditions, are needed to diagnose patellar instability clinically.

X-rays are often used to confirm a dislocated patella (image 10) when the individual presents with knee deformity. If the individual presents with a history of first-time patellar dislocation that has already been relocated, X-rays should only be ordered if there is suspicion of knee fracture, as routine images do not influence treatment in this group of patients.

Magnetic Resonance Imaging (MRI) can be used to identify bone bruising patterns consistent with recent patellar dislocation (image 7), cartilage injury and medial patellofemoral ligament (MPFL) integrity, while computed tomography (CT) scans have high diagnostic accuracy for bone fracture. However, in the absence of clinical or X-ray findings indicating surgery, MRI and CT do not influence treatment after first-time dislocation and are therefore not necessary.

Image 10: lateral patellar dislocation on X-ray.

Imaging

If surgery is indicated, imaging can be used to identify anatomical risk factors and the presence of associated knee injuries, which are used to guide surgical treatment.

Considering patellar dislocations occur most often in the second decade of the life, X-rays will identify whether the individual’s growth plates (physes) have closed, as open growth plates contraindicate certain surgical procedures.

True lateral view X-rays at 20-30° of knee flexion (image 11), MRI and CT can be used to calculate the patella height ratio. The patellar height ratio compares the length of the patella and its distance from a specific point on the tibia to determine whether patella alta is present. This can be measured using various methods including the Insall-Salvati ratio (image 13: A÷B) and Caton-Deschamps index (image 13: C÷D). Although there is variability between different imaging modalities, the following thresholds are used to categorise patellar height:

Measurement

Patella baja/infera

Normal

Patella alta

Insall-Salvati ratio

<0.8

0.8–1.2

>1.2

Caton-Deschamps index

<0.6

0.6–1.2

>1.2

Images 11-13: true lateral X-ray, illustration of a normal knee and method of measuring patellar height. Adapted from Arendt et al (2017): these images are generously provided by the authors as property of the University of Minnesota.

Image 14-17: DeJour classification of trochlear dysplasia. Black arrows indicate the relevant imaging findings for types A-D. Adapted from Arendt et al (2017): these images are generously provided by the authors as property of the University of Minnesota.

Skyline X-ray views at 20-30° of knee flexion, CT and MRI allow measurement of the trochlear depth (Image 18: [(A+C) ÷ 2] – B) and the sulcus angle (Image 18, angle between D and E). The trochlea is considered shallow, and therefore dysplastic, if the depth is less than 3mm or the sulcus angle is greater than 145°.

MRI provides the most accurate measurement of the TT-TG distance (Image 19, distance C) using tendon and cartilage landmarks, while CT scans provide the most accurate measurements using bony landmarks. TT-TG distances differ between CT and MRI, with MRI values measuring smaller. A recent study found that all patients with PFJ instability had values less than 18mm on MRI, whereas a TT-TG distance above 20mm on CT scan has traditionally been considered the threshold for an abnormal measurement.

Images 18-19: reference points for measuring trochlear depth and sulcus angle, and TT-TG distance. These images are generously provided by the authors as property of the University of Minnesota.

Treatment

Patellar dislocations may be managed with or without surgery. Following first-time patellar dislocation, surgery reduces the rate of re-dislocation when compared with non-surgical management (24% versus 35%) but there is no difference in long-term function, patient satisfaction or recurrent instability (∼33%). With specific reference to children and adolescents, surgery improves sports and quality of life outcomes despite providing no difference in pain, symptoms or function in daily living. Surgery is associated with specific surgical risks, depending on the procedure performed, and non-surgical management is therefore recommended for most first-time patellar dislocations.

Non-surgical management:

Immobilising the knee in a plaster of Paris, or patellar bracing (images 20-22), has been recommended following patellar dislocation but there is no clear evidence to support the use of these modalities. Limited evidence suggests a posterior splint may reduce the re-dislocation rate, while a dynamic brace may provide support to the patella within the first 30° of knee flexion.

Once the initial impairments (e.g. swelling, pain and reduced range of movement) have resolved, exercise therapy is recommended to improve patellar stability and limb alignment whilst moving. Historically, strengthening exercises that attempt to bias the inner quadriceps muscle (vastus medialis oblique) have been advocated but recent evidence suggests these muscles cannot be preferentially targeted, and there is no clinically relevant difference between these exercises and general quadriceps strengthening exercises for first time dislocations.

Control of lower limb rotation during weight bearing knee flexion, and joint position (proprioceptive) exercises have also been suggested but this has been more adequately researched in patellofemoral pain rather than patellofemoral instability. Video 3 demonstrates examples of rotational control exercises for patellar instability, which target the hip/pelvis, knee and ankle/foot.

It is important to note that the optimal exercise therapy for patellar dislocation has not been determined. In the early phases following dislocation, open chain knee extension may produce excessive lateral translational forces on the patella and negatively affect healing of the medial soft tissues. Considering patellar dislocations are more likely to occur while the knee is relatively straight, it appears logical to ensure exercises are eventually performed within these vulnerable ranges.

Video 3: rehab examples for patellar instability.

Surgery

Surgery is recommended in first-time patellar dislocations if the individual has sustained significant cartilage and/or bone injury, or for recurrent patellar dislocations despite appropriate non-surgical management. The appropriate surgical intervention is dictated by the presentation and may involve fixation of an osteochondral or avulsion (pull off) fracture, ligament reconstruction or additional procedures to address anatomical risk factors identified by imaging.

In recurrent patellar dislocation, surgery is dictated by the following criteria:

In additional to the rare but potentially serious risks of surgery (e.g. infection, DVT, death), recurrent dislocation and recurrent instability, specific operations are associated with specific risks as described below.

Post-operative protocols are often dictated by the consultant performing the surgery, therefore any restrictions that are placed on the patient should be clearly communicated between the surgeon and therapist.

Please check back here at a later date for examples of evidence-based protocols following isolation MPFL reconstruction and combined MPFL reconstruction, trochleoplasty, ± tibial tuberosity transfer.

About the Knee Resource

The Knee Resource is founded by two clinical knee specialists Richard Norris & Daniel Massey. Together they created The Knee Resource to assist healthcare professionals to make better decisions and provide patients with information and guidance about their knee problem.